Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In PEM-type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements, sometimes referred to as the gas diffusion media (DM) components, that: (1) serve as current collectors for the anode and cathode; (2) contain appropriate openings therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts; (3) remove product water vapor or liquid water from electrode to flow field channels; (4) are thermally conductive for heat rejection; and (5) have mechanical strength. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (e.g., a stack) depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack and are commonly arranged in series. Each cell within the stack comprises the MEA described earlier, and each such MEA provides its increment of voltage.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions, including proper water management and humidification, and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
Examples of technology related to PEM and other related types of fuel cell systems can be found with reference to commonly-assigned U.S. Pat. Nos. 3,985,578 to Witherspoon et al.; 5,272,017 to Swathirajan et al.; 5,624,769 to Li et al.; 5,776,624 to Neutzler; 6,103,409 to DiPierno Bosco et al.; 6,277,513 to Swathirajan et al.; 6,350,539 to Woods, III et al.; 6,372,376 to Fronk et al.; 6,376,111 to Mathias et al.; 6,521,381 to Vyas et al.; 6,524,736 to Sompalli et al.; 6,528,191 to Senner; 6,566,004 to Fly et al.; 6,630,260 to Forte et al.; 6,663,994 to Fly et al.; 6,740,433 to Senner; 6,777,120 to Nelson et al.; 6,793,544 to Brady et al.; 6,794,068 to Rapaport et al.; 6,811,918 to Blunk et al.; 6,824,909 to Mathias et al.; U.S. Patent Application Publication Nos. 2004/0229087 to Senner et al.; 2005/0026012 to O'Hara; 2005/0026018 to O'Hara et al.; and 2005/0026523 to O'Hara et al., the entire specifications of all of which are expressly incorporated herein by reference.
During the fuel cell stack subzero start-up, at some point in time a subzero coolant stream needs to be introduced to the stack to avoid stack MEA overheating. The flowing cold coolant is heated up by the waste heat generated by the stack. In order to avoid quenching the stack, coolant is generally introduced at a relatively low flow rate, or via coolant pulsing to avoid coolant flow poor distribution. Such an operating strategy can cause a large down-the-channel temperature variation within a cell, because the cold coolant stream will quench the section of the plate near the coolant inlet, while the amount of coolant does not have enough cooling capacity to control the temperature of the plate near the coolant outlet. For example, more than 50° C. in temperature variation along the bipolar plate has been frequently observed in experiments for such start-up operations. Experimental results seem to indicate that when coolant overcools a region of the stack near the coolant inlet, the current density is driven from the cold portion to the warm portion, which further exacerbates cold zones (i.e., no longer being heated due to lack of current density) and hot zones (i.e., heated at an even greater rate due to increased current density).
Such a wide temperature distribution within a cell can result in poor distribution of RH, in that the membrane at the high temperature zone is relatively dry while the membrane at the low temperature zone is very wet, thus reducing the MEA durability. In addition, such a wide temperature distribution could have negative impact on the mechanical stress of the bipolar plate, MEA, DM and/or the like.
Attempting to heat the stack without coolant flow shifting has resulted in large temperature variations within the stack, as shown in FIG. 1. In this view, the stack, at start-up, is represented by ten elements, with an overall temperature variation of approximately 50° C., e.g., between element 1 and element 10.
An alternative to conventional approaches has been to use an in-line coolant heater, for example, a heater in the coolant manifold of the stack which might be able to reduce the down-the-channel temperature variation within a cell. However, the mechanization utilizing the in-line heater will cost more, and will consume power, thus resulting in lower fuel efficiency.
Accordingly, there exists a need for new and improved fuel cell systems, for example those having coolant flow shifting systems operable during freeze start-up conditions so as to promote stack durability and fast start-up.